The invention relates generally to direct digital synthesizers and in particular to a direct digital synthesizer utilizing band-shaped dither cancellation to reduce spurious signals.
An existing Direct Digital Synthesizer (DDS) is depicted in FIG. 1. As shown in FIG. 1, the DDS includes a phase accumulator 10 that receives an input corresponding to a change in phase, xcex94xcex8. The phase accumulator maintains a running total of the phase. The current phase is output to a phase-to-amplitude converter 14 that generates an amplitude value based on the current phase. The phase-to-amplitude converter generates a representation of a sine wave amplitude based on the phase. This may be performed using a look-up table or other known techniques. Lastly, a digital-to-analog converter (DAC) 16 converts the digital amplitude values to an analog signal to generate a sine wave.
The DDS of FIG. 1 suffers from a high level of noise in the output signal, often referred to as spurious signals or spurs. These spurs may be caused by numerical truncation errors and DAC errors. The spurious signal spectrum can change dramatically with slight changes in the input making filtering difficult.
Different approaches have been implemented to address the spurious signals generated by the basic DDS of FIG. 1. Existing DDS devices incorporate frequency, phase or amplitude dither in an attempt to destroy the coherent nature of the DDS spurious sources. These existing dither techniques produce a high level of output noise, are targeted at reducing circuit complexity rather than improving spurious signals, or are of limited effectiveness in reducing DAC generated spurious signals.
Yet another approach involves filtering the output of the DDS using additional RF hardware to reduce spurious signals. Essentially, a DDS is followed by additional RF circuitry (e.g., dividers, PLL""s) to xe2x80x9cclean upxe2x80x9d the output spectrum of the DDS. These techniques require a substantial amount of additional RF hardware, and/or limit the modulation capabilities of the DDS.
An exemplary embodiment of the invention is a direct digital synthesizer for generating an output signal within a frequency band. The direct digital synthesizer comprises an input section for receiving a phase differential value and generating a phase angle value. A phase-amplitude converter generates an amplitude value in response to the phase angle value. A band-shaped dither generator generates a dither value. A first combiner sums the amplitude value and the dither value to define a first combined value. A second combiner differences the amplitude value and the dither value to define a second combined value. A first digital-to-analog converter (DAC) converts the first combined value to a first analog signal. A second digital-to-analog converter (DAC) converts the second combined value to a second analog signal. An output combiner combines the first analog signal and the second analog signal to generate the output signal.